Silicon carbide (SiC) has a broader band gap and a higher dielectric breakdown field strength than silicon (Si), and therefore, is expected to be the best semiconductor material to make a next-generation low-loss power device. Silicon carbide has a lot of poly-types including cubic ones such as 3C-silicon carbide and hexagonal ones such as 6H-silicon carbide and 4H-silicon carbide. Among these various poly-types, the one that is used generally to make a practical silicon carbide semiconductor device is 4H-silicon carbide.
Among various power devices that use silicon carbide, field-effect transistors such as a metal-insulator-semiconductor field-effect transistor (which will be hereinafter referred to as a “MISFET”) and a metal-semiconductor field-effect transistor (which will be hereinafter referred to as a “MESFET”) are known as typical switching elements. And a metal-oxide-semiconductor field-effect transistor (which will be hereinafter referred to as a “MOSFET”) is one of those MISFETs.
Such a switching element can switch between ON state in which drain current of several amperes (A) or more flows and OFF state in which the drain current becomes zero by changing the voltage applied to between its gate and source electrode. Also, in the OFF state, such a switching element will achieve as high a breakdown voltage as several hundred volts or more.
As for rectifiers, a Schottky diode, a pn diode and other silicon carbide rectifiers have already been reported and are all expected to be rectifiers that can operate with a huge amount of current and with a high breakdown voltage.
Silicon carbide has a higher dielectric breakdown field strength and a higher thermal conductivity than Si. That is why a power device that uses silicon carbide (which will be hereinafter referred to as a “silicon carbide power device”) can have a higher breakdown voltage and will cause smaller loss than a Si power device. That is why if a silicon carbide power device can have as high performance as a Si power device, the silicon carbide power device can have much smaller area and thickness than the Si power device.
To make an even larger amount of current flow through a power device such as a MISFET, it is effective to increase the number of devices integrated per unit area. For that reason, a vertical power MISFET with a trench gate structure has been proposed as a replacement for a conventional planar gate structure. In a MISFET with the trench gate structure, a channel region is defined on the side surface of a trench which in formed in a semiconductor layer. That is why the unit cell area can be reduced and the number of devices integrated per unit area can be increased.
A vertical MOSFET with such a trench gate structure will now be described as a conventional semiconductor device.
In the conventional semiconductor device, a silicon carbide layer including an N-type drift region and a P-type body region has been formed on a substrate of silicon carbide. An N-type source region is defined in a portion of a surface region of the body region. A trench has been cut to penetrate the source region and the body region and reach the drift region. A gate insulating film has been formed to cover the side surface and bottom of the trench. And a gate electrode has been formed on the gate insulating film so as to fill the trench. A source electrode is arranged on the silicon carbide layer to contact with the source region and the body region. And a drain electrode is arranged on the back surface of the substrate.
In such a vertical MOSFET, when a high voltage is applied to between its source and drain, an electric field easily gets overconcentrated at the bottom of the trench, which is a problem, because the gate insulating film could cause dielectric breakdown at the bottom of the trench. Thus, to avoid such a situation, people proposed that the electric field applied to the bottom of the trench be reduced by defining a P-type region in a portion of the silicon carbide layer which is located under the bottom of the trench. For example, some people proposed that a P-type region be defined by implanting P-type dopant ions into a silicon carbide layer and then a trench be cut to reach the P-type region (see Patent Document No. 1). Meanwhile, other people proposed that a trench be formed in a silicon carbide layer and then a P-type region be defined by implanting P-type dopant ions into the silicon carbide layer through the bottom of the trench (see Patent Document No. 2).